1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with a perpendicular magnetic recording layer for use in magnetic recording hard disk drives.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording system is one that uses a “dual-layer” media. This type of system is shown in FIG. 1 with a single write pole type of recording head. The dual-layer media includes a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field Hw acting on the recording layer RL. The disk also includes the hard disk substrate, a seed or onset layer (OL) for growth of the SUL, a nonmagnetic exchange break layer (EBL) to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and to facilitate epitaxial growth of the RL, and a protective overcoat (OC). As shown in FIG. 2, the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole (FIG. 1) which is the real write head (RWH) above the disk, and an effective secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the EBL and by virtue of its high permeability produces a magnetic mirror image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field Hw inside the RL.
One type of material for the RL is a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce inter-granular exchange coupling, which is responsible for high intrinsic media noise. Thus, enhancement of grain segregation in the cobalt alloy RL has been proposed by the addition of oxides, including oxides of Si, Ta and Nb, which precipitate to the grain boundaries.
A perpendicular magnetic recording medium has also been proposed wherein the RL is an antiferromagnetically-coupled (AFC) recording layer of two ferromagnetic layers, each having perpendicular magnetic anisotropy, separated by an antiferromagnetically (AF) coupling layer. The AF-coupling layer induces perpendicular antiferromagnetic exchange coupling between the two ferromagnetic layers, as depicted in FIG. 3 by the antiparallel magnetization directions between the two ferromagnetic layers in each magnetized region of the AFC RL. The upper ferromagnetic layer is formed with a higher magnetic moment than the lower ferromagnetic layer, typically by making it thicker, so that the AFC RL has a net magnetic moment in the absence of a magnetic field. In this type of medium, as described in U.S. Pat. No. 6,815,082 B2, both the first or lower ferromagnetic layer and the second or upper ferromagnetic layer are formed of a granular cobalt alloy, such as a CoPtCr alloy, with an hcp crystalline structure and with or without oxides.
The cobalt alloy RL, including the cobalt alloy AFC RL, with or without oxides, has out-of-plane of perpendicular magnetic anisotropy as a result of the c-axis of its hcp crystalline structure being induced to grow perpendicular to the plane of the layer during deposition. To induce this growth of the hcp RL, the EBL onto which the RL is formed is also an hcp material. In a perpendicular magnetic recording medium with an AFC RL, the EBL also has an hcp crystalline structure to induce the perpendicular magnetic anisotropy of the lower layer in the AFC RL. Ruthenium (Ru) is one type of nonmagnetic hcp material proposed for the EBL. While not shown in FIG. 2, a seed layer is typically deposited directly on the SUL to facilitate the growth of the EBL.
To achieve high performance perpendicular magnetic recording disks at ultra-high recording densities, e.g., greater than 200 Gbits/in2, the RL should exhibit low intrinsic media noise (high signal-to-noise ratio or SNR), a coercivity Hc greater than about 5000 Oe and a nucleation field Hn greater (more negative) than about −1500 Oe. The nucleation field Hn has several meanings, but as used herein it is the reversing field, preferably in the second quadrant of the M-H hysteresis loop, at which the magnetization drops to 90% of its saturation value Ms. The more negative the nucleation field, the more stable the remanent magnetic state will be because a larger reversing field is required to alter the magnetization.
A perpendicular magnetic recording medium with a RL of a CoPtCr granular alloy with added SiO2 is described by H. Uwazumi, et al., “CoPtCr—SiO2 Granular Media for High-Density Perpendicular Recording”, IEEE Transactions on Magnetics, Vol. 39, No. 4, July 2003, pp. 1914-1918. The RL had Hc of about 4000 Oe and Hn of about −700 Oe. A perpendicular magnetic recording medium with a RL of a CoPt granular alloy with added Ta2O5 is described by T. Chiba et al., “Structure and magnetic properties of Co—Pt—Ta2O5 film for perpendicular magnetic recording media”, Journal of Magnetism and Magnetic Materials, Vol. 287, February 2005, pp. 167-171. The RL had Hc of about 3000 Oe.
In perpendicular magnetic recording systems the recording medium is part of the write head and therefore needs to match with the head design, as depicted in FIG. 2 for a system with a single-write-pole head. For a single-write-pole head, it is desirable to minimize the write-pole-to-SUL spacing to concentrate the write field flux and thus maximize the write field. Another type of system uses a trailing-shield type of write head. In this system the write-pole-to-trailing-shield distance should match with the write-pole-to-SUL spacing to obtain the best write angle. In both systems, a thin EBL is used to achieve the desired head-to-SUL spacing. However, while a reduction in thickness of the EBL is desirable for writing, the EBL is made thick enough to provide the template for the growth of the hcp cobalt alloy RL to assure that its c-axis is perpendicular. The EBL is also sufficiently thick to provide an RL with high Hc and low enough inter-granular exchange coupling to minimize the intrinsic media noise. The thickness of the EBL required for RLs with Si oxides is typically greater than about 20 nm. The RL with Ta oxides reported in the above-cited article by T. Chiba et al. had a Ru EBL with a thickness of 15 nm.
What is needed is a high-performance, ultra-high-recording-density perpendicular magnetic recording disk with a CoPtCr granular alloy RL and a thin EBL for optimal write performance.